1. Field of the Invention
The present invention relates to methods and apparatus for stabilizing output beam parameters of a gas discharge laser. More particularly, the present invention relates to a gas discharge laser system which includes components for monitoring a corona discharge ignition voltage at an electrostatic dust precipitator, to provide gas mixture status information, which is preferably for guiding gas control actions for maintaining an optimal gas mixture composition over long periods.
2. Description of the Related Art
Pulsed gas discharge lasers such as excimer and molecular flourine lasers, emitting in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) have become very important for industrial applications such as photolithography. Such lasers generally include a discharge chamber containing two or more gases such as a halogen-containing species and one or two rare gases. KrF (248 nm), ArF (193 nm), XeF (350 nm), KrCl (222 nm), XeCl (308 nm), and F2 (157 nm) lasers are examples.
Efficiencies of excitation of the gas mixtures and various parameters of output beams of these lasers vary sensitively with the compositions of their gas mixtures. An illustrative gas mixture composition for a KrF laser may have gas mixture component ratios around xcx9c0.1% F2/xcx9c1.0% Kr/xcx9c98.9% Ne (see U.S. Pat. No. 4,393,505, which is assigned to the same assignee and is hereby incorporated by reference). For an ArF laser, an around 1.0% concentration of argon would be used instead of the around 1.0% krypton of the KrF laser. A F2 laser may have a gas component ratio around xcx9c0.1% F2/xcx9c99.9% He and/or Ne (see U.S. Pat. No. 6,157,662, which is assigned to the same assignee as the present application and is hereby incorporated by reference). Small amounts of a gas additive, e.g., Xe, may be added to any of these gas mixtures for improving energy stability or overshoot control, for example. (see U.S. patent application Ser. No. 09/513,025, which is assigned to the same assignee as the present application and is hereby incorporated by reference; see also R. S. Taylor and K. E. Leopold, Transmission Properties of Spark Preionization Radiation in Rare-Gas Halide Laser Gas Mixes, IEEE Journal of Quantum Electronics, pp. 2195-2207, vol. 31, no. 12 (December 1995)). Any deviation from the optimum gas compositions of these lasers typically results in instabilities or reductions from optimal of one or more output beam parameters such as beam energy, energy stability, temporal pulse width, temporal coherence, spatial coherence, discharge width, bandwidth, and long and short axial beam profiles and divergences.
Especially important in this regard is the concentration (or partial pressure) of the halogen-containing species, e.g., F2 or HCL, in the gas mixture. FIG. 1 shows laser output efficiency versus fluorine concentration for a KrF-excimer laser, illustrating a decreasing output efficiency away from a central maximum. FIG. 2 shows the dependence of output energy on driving voltage (i.e., applied by the discharge circuit to the gas mixture at electrodes within a discharge chamber of the laser). FIG. 3A illustrates effects of gas mixture aging on laser output energy. FIG. 3B further illustrates how the slope of the curve for energy output vs. driving voltage also decreases with aging of the gas mixture.
For industrial applications, it is recognized in the present invention that it would be advantageous to have an excimer or molecular fluorine laser capable of operating continuously for long periods of time, i.e., having minimal downtime. It is desired to have an excimer or molecular fluorine laser capable of running non-stop year round, or at least having a minimal number and duration of down time periods for scheduled maintenance, while maintaining constant output beam parameters. For example, uptimes of greater than 95% or even 98% wold be advantageous and may be achieved if precise control and stabilization of output beam parameters, including precise control of the composition of the gas mixture were provided with these laser systems.
Unfortunately, gas contamination occurs during operation of excimer and molecular fluorine lasers due to the aggressive nature of the fluorine or chlorine-containing species in the gas mixture. The halogen gas is highly reactive and its concentration in the gas mixture decreases as it reacts, leaving traces of contaminants. The halogen gas reacts with materials of the discharge chamber or tube as well as impurities in the chamber. Moreover, the reactions take place and the gas mixture degrades whether the laser is operating (discharging) or not. The passive gas lifetime is about one week for a typical KrF-laser.
During operation of a KrF-excimer laser, such contaminants as HF, CF4, COF2, SiF4 have been observed to increase in concentration rapidly (see G. M. Jurisch et al., Gas Contaminant Effects in Discharge-Excited KrF Lasers, Applied Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)). For a static KrF laser gas mixture, i.e., with no discharge running, increases in the concentrations of HF, O2, CO2 and SiF4 have been observed (see Jurisch et al., above).
One way to reduce the rate of this gas degradation is by reducing or eliminating contamination sources within the laser discharge chamber. With this in mind, an all metal, ceramic laser tube has been disclosed (see D. Basting et al., Laserrohr fxc3xcr halogenhaltige Gasentladungslaserxe2x80x9d G 295 20 280.1, Jan. 25, 1995/Apr. 18, 1996 (disclosing the Lambda Physik Novatube, and hereby incorporated by reference into the present application)). Gas purification systems, such as cryogenic gas filters (see U.S. Pat. Nos. 4,534,034, 5,136,605, 5,430,752, 5,111,473 and 5,001,721, which are hereby incorporated by reference) or electrostatic particle filters (see U.S. Pat. Nos. 4,534,034 and 5,586,134, which are hereby incorporated by reference) may also be used to extend excimer and molecular fluorine laser gas lifetimes to, e.g., 100 million shots before a new fill of the gas mixture into the laser tube may become advisable.
It is not easy to directly measure the halogen concentration within the laser tube for making rapid online adjustments (for example, see U.S. Pat. No. 5,149,659, disclosing monitoring chemical reactions in the gas mixture, which is hereby incorporated by reference). A more preferable approach may be to indirectly monitor the halogen concentration by monitoring a parameter that varies with a know relationship to the halogen concentration. In such a method, precise values of the parameter would be directly measured, and the F2 concentration would be calculated from those values or pulled from tables stored in a memory accessible by a control processor of the laser system. In this way, the F2 concentration may be indirectly monitored (see U.S. patent application Ser. No. 09/734,459, which is assigned to the same assignee as the present application and is hereby incorporated by reference, disclosing indirect monitoring of the composition of the gas mixture by monitoring laser input and/or output beam parameters).
Some methods have been disclosed for such indirect monitoring of halogen depletion in a narrow band excimer laser by monitoring beam profile (see U.S. Pat. No. 5,642,374, hereby incorporated by reference) and spectral (band) width (see U.S. Pat. No. 5,450,436, hereby incorporated by reference). However, beam profile and spectral width are each influenced by various other operational conditions such as repetition rate, tuning accuracy, thermal conditions and aging of the laser tube. Thus, the same spectral width can be generated by different gas compositions depending on these other operating conditions.
Another way of stabilizing, during operation, a gas mixture with a gas composition initially provided within a discharge chamber of an excimer or molecular fluorine gas discharge laser is described in U.S. Pat. No. 6,243,405, which is assigned to the same assignee as the present application and is hereby incorporated by reference. The method disclosed in the ""818 application includes monitoring a temporal pulse shape of the laser beam and adjusting and/or determining the status of the gas mixture based on the monitored temporal pulse shape. The monitored temporal pulse shape is preferably compared with a reference temporal pulse shape. A difference or deviation between a measured temporal pulse shape and a reference temporal pulse shape is determined. The amounts of and intervals between gas replenishment actions are determined based on the pulse shape deviation. The energy of the beam is preferably also monitored and the driving voltage and gas actions are adjusted to stabilize the energy, energy stability and/or energy dose.
Another advantageous technique has been disclosed including monitoring amplified spontaneous emission (ASE), and is described in U.S. Pat. No. 6,243,406, which is assigned to the same assignee as the present application and is hereby incorporated by reference. The ASE is demonstrated to be very sensitive to changes in fluorine concentration, and thus the fluorine concentration may be monitored indirectly by monitoring the ASE, notwithstanding whether other parameters are changing and effecting each other as the fluorine concentration in the gas mixture changes.
Another way to measure the gas status is to use a mass spectrometer (see U.S. Pat. No. 5,090,020 and hereby incorporated by reference). However, this device is costly to incorporate into an excimer laser system. The gas status may also be monitored by U-I characteristics of a corona discharge (see German Patent Application DE 42 22 418 A1, which is hereby incorporated by reference). However, it is desired to have a method that has a higher sensitivity.
Excimer lasers have utilized gas flow loops for removing gases from the laser tube, cleaning the removed gas by passing it through an electrostatic precipitator, and putting the gas back into the laser tube by passing the clean gas near the laser windows and through a baffle structure which keeps the laser tube windows clean. Some early designs are described at U.S. Pat. Nos. 4,534,034 and 5,018,162 which are hereby incorporated by reference, as well as U.S. Pat. No. 5,729,564 which describes a later design. In addition, Lambda Physik AG of Goettingen, Germany and Fort Lauderdale, Fla. has been selling excimer lasers for many years having a pair of gas flow loops which draw gas from the laser tube near the respective laser tube windows, pass the gas through the precipitator to clean the gas, and re-insert the gas by flowing past the windows and through baffle boxes and into the main volume of the laser tube. These techniques lengthen the lifetime of the laser tube windows and the gas mixture. A technique is provided in the present invention for further enhancing the lifetime of the windows and/or the gas mixture.
In view of the above, it is an object of the invention to provide a technique for stabilizing output beam parameters affected by halogen depletion by controlling the halogen and/or rare gas concentrations in the gas mixture within the laser tube of an excimer or molecular fluorine (F2) laser. The present invention provides an advantageous technique for determining the status of the gas mixture in the discharge chamber. The status information can then be used to control the halogen and/or rare gas concentrations.
It is a further object of the invention to provide a technique for monitoring gas mixture status that independently monitors when and to what extent the mixture has xe2x80x98agedxe2x80x99 (meaning changes in the concentration of one or more component gases over time). The present invention provides an advantageous technique for determining the status of the gas mixture which is not affected by resonator losses or degradation of the optics. The invention provides a signal which is independently related to the status of the gas mixture of the gas discharge laser. This sign al may also be used as an input to an algorithm, or a control circuit, to better guide performance of gas replenishment actions. The invention would be particularly useful in all gas lasers where sustained optimum performance requires compensating for the xe2x80x9cagingxe2x80x9d of the active medium. The invention would be especially useful in pulsed gas discharge lasers (e.g. excimer lasers, including F2 lasers).
Specifically, an excimer or molecular fluorine laser system is provided which emits a laser beam during operation and has a gas mixture with a gas composition initially provided within a discharge chamber. The laser system includes a discharge chamber containing a laser gas mixture at least including a halogen-containing species and a buffer gas, multiple electrodes within the discharge chamber and connected to a discharge circuit for energizing the gas mixture, a resonator for generating a laser beam, an electrostatic precipitator for having a voltage applied thereto and for receiving and precipitating contaminant particulates from a flow of the gas mixture, and a processor for monitoring the voltage applied to the electrostatic precipitator and for determining a status of said gas mixture based on the voltage applied to the electrostatic precipitator. The determined status preferably includes a concentration of the halogen-containing species.
The laser system preferably further includes a gas control unit for replenishing the laser gas mixture in response to signals from the processor based on the status of the gas mixture determined from the voltage applied to the electrostatic precipitator. The gas replenishment preferably includes replenishing the halogen-containing species of the gas mixture.
An excimer or molecular fluorine laser system which emits a laser beam during operation and has a gas mixture with a gas composition initially provided within a discharge chamber is further provided including a discharge chamber containing a laser gas mixture at least including a halogen-containing species and a buffer gas, multiple electrodes within the discharge chamber and connected to a discharge circuit for energizing the gas mixture, a resonator for generating a laser beam, an electrode for having a voltage applied thereto and for generating a corona discharge in a gas mixture environment, a corona discharge ignition voltage probe for monitoring a corona discharge ignition voltage applied to the electrode, and a processor for receiving signals corresponding to the corona discharge ignition voltage monitored by the corona discharge ignition voltage probe and for determining a status of the gas mixture based on the corona discharge ignition voltage. The determined status preferably includes a concentration of the halogen-containing species.
The laser system preferably further includes a gas control unit for replenishing the laser gas mixture in response to signals from the processor based on the status of the gas mixture determined from the corona discharge ignition voltage. The gas replenishment preferably includes replenishing the halogen-containing species of the gas mixture.
A method of stabilizing output beam parameters of an excimer or molecular fluorine laser system which emits a laser beam during operation and has a gas mixture with a gas composition initially provided within a discharge chamber, wherein the laser system includes an electrostatic precipitator, is also provided including monitoring a voltage applied to the electrostatic precipitator and determining a status of the gas mixture based on the voltage applied to the electrostatic precipitator. The determining operation preferably includes determining a concentration of a halogen-containing species of the gas mixture.
The method preferably further includes the operation replenishing the gas mixture based on the status of the gas mixture determined from the voltage applied to the electrostatic precipitator. The replenishing operation preferably includes replenishing a halogen-containing species of the gas mixture.
A further method is provided for stabilizing output beam parameters of an excimer or molecular fluorine laser system which emits a laser beam during operation and has a gas mixture with a gas composition initially provided within a discharge chamber including the operations applying a voltage to an electrode for generating a corona discharge in a gas mixture environment, monitoring a corona discharge ignition voltage applied to the electrode, and determining a status of the gas mixture based on the corona discharge ignition voltage. The determined status preferably includes a concentration of a halogen-containing species of the gas mixture.
The method preferably further includes the operation replenishing the gas mixture based on the status of the gas mixture determined from the corona discharge ignition voltage. The replenishing operation preferably includes replenishing a halogen-containing species of the gas mixture.